1. Field of the Invention
This invention relates to gypsum board, and more specifically to apparatus and method for coating a cover sheet of gypsum board, and to the board produced thereby.
2. Description of Related Technology
Gypsum board is well known and widely used in the construction industry. A typical sheet of gypsum wallboard comprises a gypsum core, a back cover sheet on one side of the core and a face or front cover sheet on the other side of the core. The face cover sheet is folded around the long edges of the core and overlaps the side edges of the back cover sheet.
To reduce the weight of the core, it has been common practice to introduce small bubbles into the gypsum to produce a foamed gypsum core. This has been done, for example, by adding a foaming agent to the gypsum slurry. However, a core formed entirely of foamed gypsum has two disadvantages. First, the foamed gypsum core, when set or cured, is relatively fragile and tends to crack and crumble when a nail is driven through it during installation. Second, the foamed gypsum core does not always adhere to the cover sheets as well as desired.
To avoid the first problem mentioned above, it has been common practice to provide unfoamed gypsum along the long edges of the board. The unfoamed gypsum along the edges is denser and harder than the foamed gypsum, and the "hard edges" are stronger and less easily fractured. The second problem has been more difficult to resolve. Starch has been added to the gypsum slurry to produce better adherence with the cover sheets. Another solution has been to coat the cover sheets to produce better adhesion.
The White U.S. Pat. No. 4,327,146 describes a method of coating a cover sheet with a defoaming agent which acts to remove the foam bubbles from the gypsum at the core-sheet interface. The defoamed gypsum adheres well to the cover sheets.
The Brookby U.S. Pat. No. 1,511,500 describes a method of coating a cover sheet with a "normal" gypsum and water mixture, and then forming a core layer of gypsum having an expanded cellular body.
The R. Bruce U.S. Pat. Nos. 5,085,929 and 5,116,671 describe a method of producing a foamed gypsum slurry using a foaming agent which is added to the gypsum. According to these patents, the foaming agent produces a high density slurry at the interface with the cover sheets, and the high density slurry purportedly adheres well to the paper.
Other U.S. patents of possible interest are U.S. Pat. Nos. 2,954,302, 3,516,882, and 3,607,486.
It is a general object of the present invention to provide an improved apparatus and method for coating cover sheets with a relatively high density gypsum slurry, and to an improved gypsum board produced thereby.